1995-04-11 04:00:00 PDT Washington -- In the latest sign of the Democrats' eroding base, a conservative House member joined the Republican Party yesterday, becoming the third congressional Democrat to abandon the party since the GOP swept November's elections.

The defection of Representative Nathan Deal, D-Ga., part of a vanishing breed of conservative Democrats in the House, strengthens the Republicans' hold on the South -- a onetime Democratic stronghold that now has a GOP majority in the House.

It is also a bad omen for Democrats with any hopes for retaking control of the House in 1996. The Republican margin over Democrats in the House is now 231 to 203, with one independent.

"What a disaster for the Democrats," said Stuart Rothenberg, a congressional analyst, who said that Deal's party switch could prompt others. "This just adds to the problems they have in the region."

Deal, 52, was one of only four Democrats in the 11-member Georgia delegation, and the only white Democratic member. As recently as last year, Georgia had a seven- member Democratic majority: four white, three black.

Deal was one of two dozen or more mostly Southern conservative Democratic House members who bucked their party leadership to support most of the GOP's Contract With America. He said yesterday that his decision to change parties was "based on principle, not politics."

Deal had been courted by the Republicans since last fall, when he was included on a list of possible recruits should the GOP have fallen a few votes short of winning a House majority.

But he told the Atlanta Journal and Constitution before the election that he didn't think it would be "honest" to "run with one party label and then, after the election, change parties."

Deal made similar comments to a Georgia radio station three months ago. "If I choose to switch during the term, I think the honest thing to do is to resign and have a special election," he said.